PSI - Issue 12

G. Zucca et al. / Procedia Structural Integrity 12 (2018) 183–195

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G. Zucca et al. / Structural Integrity Procedia 00 (2018) 000–000

measures, generally aimed at reducing the stress intensification factors (eg connections), or the increase the material performances (shot peening, rolling, etc. ...)Shigley et al. (2002). By virtue of what has been said, the component is then qualified and submitted to the durability test, this enters the testing field, i.e. the tests in which the operating conditions are replicated and the life simulated until the breakdown. The tests are often very long and expensive, the duration is the critical element and the acceleration of the tests, obtained by worsening the conditions of exercise, often leads to very conservative results Niesłony et al. (2016). The challenge undertaken by the research aimed at experimentation hinges on the concertation of the objectives of speed reliability and economic sustainability of the tests, which, as often happens in engineering, are di ffi cult to reconcile. Once the project estimates have been verified, the component is then certified and based on the assessments above, the maintenance and / or controls are scheduled, identifying the critical areas, the appropriate techniques and the intervals. In a scenario where the designer is calculating the durability of a component based on an estimated exercise and the experimenter testing it on an accelerated qualification spectrumWang et al. (2013), what happens when the component arrives at the end of his life without having given signs of failure? This is clearly attributable to the safety factors assumed in the design phase and mainly, as already mentioned above, to a use that has been overestimated for reasons of opportunity. The question we asked ourselves and was the harbinger of this work was therefore: is it possible, knowing the real history of use of an object, to estimate its residual life? The reasons for this question go beyond the mere academic interest and are to be found in the economic and logistical impact that the delayed or avoided replacement of a valuable component can have in a completely transversal context. This creates a figure that is halfway between the designer and the experimenter in which the skills of design, modeling and measurement are put in place in a multidisciplinary manner, with a view to improving and refining what is already existing. As a benchmark for the activity, the case deriving from a real need occurred on an external system of suspension of loads (e.g. fuel tanks) of a fleet of aircrafts operated by the Italian Air Force, developed and realized in the ’70s, was used. The object of the study concerns the connection points between the aircraft and the system that had been built on the basis of a presumed operational use and whose duration in 4000 hours of operation was estimated. To date, after more than 25 years, the expiry of the useful life has come nearer, since the failure of the components has not been recorded, the residual life of the same has been estimated starting from a well-known and consolidated use history, developing the dissertation as follows. In the first paragraph, for what is allowed by the secrecy constraints, the system will be described giving evidence of the: archive design documentation NATO, schematization of the structure, idealization of the material and hypotheses at the base. In the second paragraph, the operational statistics and the guidelines used in the design of the experimental tests will be discussed, which will be given wide visibility of the management and results. The passage from the experimental data to the load spectra and then to the stress will be discussed in the third paragraph in which the fem model will be described and the results will be discussed. The fourth paragraph, dedicated to the estimation of durability, identifies all the critical aspects of the application of the classical theory to a real case, adopting an energetic model for the estimation of multiaxiality and proposing a hybrid method in which the analysis in the frequency domain J. S. Bendat (1964), Pitoiset and Preumont (2000), Braccesi et al. (2015) is combined to Rainflow Counting in order to reduce calculation time. The time vs frequency comparisons will then be performed by correlating the results to the statistical indicators. The possibility of suspending and transporting payloads outside an aircraft is a need felt since the dawn of the aeronautics, which even calls to mind the immediate application in the war, is widely used for the transport of various kind of pods (eg optical observation, radar) and fuel tanks Gern and Librescu (1998), Lasek and Sibilski (2002), Chen et al. (2002). The aircraft object of the study, arrived in the second half of the operational life, is equipped with a multipurpose system of suspension of external payload constituted, for the part of interest, by two pylons located in the ventral part of the fuselage (Fig. 1), able to transport di ff erent types of payloads depending on the needs.The pylons are connected to the aircraft by means of two pins (Fig. 2) that represent the real object of the analysis, at the time of the design it was in fact estimated a nominal life of these objects in 4000 flight hours, from the analysis of the archive documentation we tried to reconstruct the steps followed by the designer to achieve this result. The estimate of durability was based 2. Historical Background